Bonebreak, I was having the same problems with an Os 40 La, there were some shavings of metal inside, i went through 3 plugs in 3 flights, anyway through this site with the aid of fellow modelists, i cleaned out the engine, ran it a little richer and i've been running with the same plug for 2 weeks without a problem. Everyhting was pretty much the same as you, new engine, 15%nitro, OS no. 3 plug, i'm using an OS no. 8 and havent looked back.
Hope this helps
Will